1. Field of the Invention
The present invention relates to voltage controlled oscillators (VCOs), such as those used in phase lock loops (PLLs), and in particular, to filter circuits used to filter control voltages for controlling voltage controlled capacitances in VCOs.
2. Description of the Related Art
All radio transceiver systems require the use of one, though usually more, internal signal source, such as an oscillator, for generating the one or more internal signals needed for frequency converting, modulating or demodulating outgoing and incoming communication signals. More often than not, particularly in modern radio transceivers, such internal signal sources are implemented as frequency synthesizers using PLL circuits. As is well known in the art, PLL circuits generally include an oscillator circuit that generates the desired output signal. This output signal is fed back and compared to some form of internal reference signal, such as that generated by a crystal controlled oscillator. This comparison process can be in the form of a phase or frequency comparison of the two signals. As a result of this comparison, a control signal is generated and filtered for controlling the oscillator that provides the actual output signal. Most PLL circuits are digital PLL circuits, which means that this control signal requires significant filtering prior to its use in controlling the oscillator.
Performance requirements of modern radio transceiver systems continue to become increasingly stringent. As a result, phase noise and frequency stability performance characteristics of the PLL circuit become increasingly significant in determining the overall performance of the transceiver system. Generally, it is the VCO that is primarily responsible for determining, and often limiting, these performance criteria. In modern systems, this problem is made worse by the fact that it is generally necessary to implement the VCO within a monolithic integrated circuit (IC) to achieve low power consumption and low cost for transceivers such as those used in cellular telephones and other forms of wireless data appliance systems.
Particularly within the environment of an IC, the design of the integrated VCO often presents many challenges due to many different and conflicting performance criteria, such as phase noise, output frequency range, input tuning voltage range, output signal power and output signal frequency changes due to either load variations or power supply variations. When load variations cause the output signal frequency to change, it is considered to be xe2x80x9cpulledxe2x80x9d with respect to its desired value, whereas when power supply variations cause the output signal frequency to change, it is considered to be xe2x80x9cpushedxe2x80x9d with respect to its desired value.
Further complicating the problems of implementing an integrated VCO is the fact that implementations of passive components in an IC are significantly limited in terms of their quality and performance tolerances. Hence, the ability of a designer to isolate an integrated VCO from frequency xe2x80x9cpullingxe2x80x9d or xe2x80x9cpushingxe2x80x9d effects is limited by the quality of passive components available in an IC as are typically needed for implementing appropriate filter circuits.
Accordingly, it would be desirable to have a filter circuit that, when used to filter a control voltage for controlling a VCO, particularly within a monolithic IC environment, is substantially unaffected by the typical limitations associated with the passive components available in such an IC environment.
In accordance with the present invention, the controlling circuit element of a voltage controlled oscillator (VCO) for use in a phase lock loop (PLL) is a voltage controlled capacitance (varactor) connected between the terminals used to convey the power supply voltage and the frequency control voltage. The loop filter, implemented in a shunt configuration at the input of the VCO, is also connected between the power supply and frequency control voltage terminals. As a result, any variations in the power supply voltage appear at both terminals of the varactor due to the voltage coupling effect of the loop filter between the shared power supply and frequency control voltage terminals.
In accordance with one embodiment of the present invention, a filter circuit for filtering a control voltage used to control varactor circuitry in a VCO includes a power supply terminal, a control terminal and shunt filter circuitry. The power supply terminal conveys a power supply voltage having a magnitude and polarity relative to a reference potential. The control terminal conveys a control voltage for varactor circuitry, wherein, relative to the reference potential, the control voltage has a polarity equal to the power supply voltage polarity and a magnitude less than or equal to the power supply voltage magnitude. The shunt filter circuitry, connected between the power supply and control terminals, filters the control voltage.
In accordance with another embodiment of the present invention, also included is varactor circuitry, connected between the power supply and control terminals, that in response to the control voltage exhibits a voltage controlled capacitance. Such varactor circuitry can include circuit elements such as a diode or an insulated gate field effect transistor.
In accordance with still another embodiment of the present invention, also included is a reference terminal that establishes the reference potential, and VCO circuitry, connected between the power supply, reference and control terminals, that in response to the control voltage generates a voltage controlled oscillation signal. Such oscillator circuitry can include bias and resonant circuitry. The bias circuitry, connected between the power supply and reference terminals, in response to the power supply voltage, generates a bias signal. The resonant circuitry, connected between selected ones of the power supply terminal, reference terminal, control terminal and bias circuitry, in response to the bias signal and control voltage generates the voltage controlled oscillation signal. The resonant circuitry can include varactor circuitry, connected between the power supply and control terminals, that in response to the control voltage exhibits a voltage controlled capacitance. Such varactor circuitry can include circuit elements such as a diode or an insulated gate field effect transistor.